Solidify PHUITwoColumnView as a page layout

Summary:
Rolls out a new "Object Page" design with PHUITwoColumnView. This is reasonably polished, but wanted to post it up for you now for feedback before chasing down minor bugs. This implements TwoColumn in the following applications:

 - Ponder
 - Paste
 - Slowvote
 - Countdown
 - Projects
 - Profile
 - Passphrase

This helped track down display issues and inconsistencies and make sure the layout was flexible for different pages.

Test Plan:
Test each of the applications on mobile, tablet, and desktop breakpoints.

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Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15366

src/applications/celerity/postprocessor/CelerityDefaultPostprocessor.php

@@ -192,10 +192,7 @@ final class CelerityDefaultPostprocessor
       'sh-disabledbackground'   => '#f3f3f3',
 
       // Background color for "most" themes.
-      'page.background' => '#f1f1f4',
-
-      // Background color for "dark" themes.
-      'page.background.dark' => '#ebecee',
+      'page.background' => '#f8f8fb',
 
       'menu.profile.text' => 'rgba(255,255,255,.8)',
       'menu.profile.text.selected' => 'rgba(255,255,255,1)',